Compounds containing an amino functionality can be oxidized with hydrogen peroxide, sometimes with an additive such as a catalyst in order to promote the reaction or sometimes in the absence of a catalyst, depending on the inherent reactivity of the substrate. Many papers or patents describe variations to the reaction between amines and hydrogen peroxide. For example, U.S. Pat. No. 4 247 480 to Nissan Chem Ind KK suggests the addition of a chelating agent selected from EDTA or stannate together with carbonic acid or a carbonate.
Disadvantageously, however, there is a tendency for nitrosamines to be formed as byproducts in reaction mixtures which contain amines, particularly secondary or tertiary amines and hydrogen peroxide. This is inevitably the case during reaction between those two reactants but it can also occur if such an amine is itself a reaction product or intermediate. For example, when tertiary amines are being converted to amine oxides by reaction with hydrogen peroxide, there is a distinct tendency for nitrosamines to be produced as a byproduct. The formation of nitrosamines is disadvantageous for several reasons, of which the most significant identified to date is the fact that many nitrosamines are carcinogenic. At least some of the nitrosamines are quite volatile, so that either in the course of the reaction or subsequently during storage or use of the main product, such volatile nitrosamines can be released into the atmosphere and thus can be inhaled by persons in the vicinity. They can be absorbed through the skin by dermal contact. This can be particularly significant for some amino derivatives such as amine oxides, in that one of their major uses is in personal hygiene products, such as shampoos or cleansing lotions which are skin contact products. Consequently, it is highly desirable to find ways of inhibiting the formation of such carcinogenic byproducts to preserve the health of chemical process operators and any subsequent user of the products.
As a second disadvantage, though of rather less importance, a number of the byproduct nitrosamines are brightly colored, such that their presence, even in low amounts, tends to impart that color to the main product. If as is the case with amine oxides, the product would otherwise be white, then the presence of the byproduct can render the product less desirable commercially, by failing to meet color specifications.
It has been observed that the peroxide/amine reaction can be sensitive to the presence of a number of interfering substances including metals which can promote by-reactions. Such substances can be present as impurities in the reactants, possibly as a result of the method of their manufacture, or introduced in water added as diluent in the reaction mixture, or can be extracted from the pipework or walls of the reaction vessels or holding tanks or can result from inadvertant ingress of foreign bodies such as dust. The concentration of such substances correlates to a reasonable extent with the extent of nitrosamine formation. Herein, when high levels of such interfering substances are present, though still within the range that can be encountered in practice, the conditions are sometimes described as "stress conditions".
In U.S. Pat. No. 5 023 376 (Shehad et al) and the corresponding EP-A-0 409 043 (Interox America), it has already been proposed to conduct amine/hydrogen peroxide reactions in the presence of a specified selection of aminopolymethylenephosphonate compounds. Together with other beneficial effects, it was found that their addition, in appropriate concentrations, resulted in the measurable reduction in the rate and/or extent of formation of nitrosamines. It can therefore be deduced from the data disclosed therein that at least some transition metal ions are in some way implicated in the mechanism for nitrosamine formation. The problem was ameliorated by chelation with the selected phosphonates, but either to a lesser extent or not appreciably by employing other organophosphonates outside the selection. From the foregoing, it was deduced that some further mechanism may be operating to generate nitrosamines or that there was some hitherto unidentified factor within the existing metal ion cataylzed route for nitrosamine formation that was contributing to the by-product formation.